This invention relates to a light-emitting semiconductor device, or light-emitting diode (LED) in common parlance, and more specifically to an LED featuring provisions for protection against overvoltages. The invention also specifically concerns a method of fabricating such an overvoltage-protected LED.
A host of specialists in electroluminescence have focused their attention in recent years on nitride semiconductors as LED materials. LEDs built from these materials emit light in the wavelength range of 365-550 nanometers. These devices have, however, an inherent weakness in withstanding electrostatic breakdown, being susceptible to destruction when subjected to a voltage surge in excess of 100 volts. It might be contemplated to incorporate a discrete overvoltage or surge protector such as a diode or capacitor in one and the same package with the LED. This solution is unsatisfactory in consideration of the greater bulk of the resulting device caused by the addition of the discrete overvoltage protector.
More sophisticated solutions are found in U.S. Unexamined Patent Publication US-2005-0168899-A1. In one of the embodiments disclosed in this prior application, an overvoltage protector diode is built into the substrate of the LED and electrically connected reversely in parallel with the light-generating semiconductor layers. The overvoltage protector diode conducts when the LED is reverse biased, limiting the cathode-anode voltage of the LED to its forward voltage. However, the forward voltage of the overvoltage protector diode (voltage at which it is triggered into conduction) is as low as one volt or even less, so that the LED can withstand a correspondingly low reverse voltage. This prior art overvoltage-protected LED does not lend itself to use in applications (e.g., LED matrix) where it is required to withstand a higher reverse voltage.
The U.S. patent application cited above also teaches to provide an npn overvoltage-protector by creating a p- and an n-type semiconductor layer in preselected parts of an n-type silicon substrate by doping. The two additional dopings required made the fabrication of the overvoltage-protected LED unnecessarily time-consuming and costly.